Are we talking about modern vehicles? Or old ones? I mean, that's a pretty big area, and you can't really compare the technology of the 40's and 50's with today's.
Here's MY votes:
Worst tank (pre-1950) - MAUS (too heavy, too slow, too restriced)
Best tank (pre-1950) - T34/85 (arguably best tank of WWII, easy and cheap to manufacture, a dummy could operate it effectively, reasonably well armored, automotively reliable, very combat effective)
Worst Tank (post-1950) - M60A2 (too far ahead of it's time, dangerous for the crew, too large, used the faulty 155mm Sheleileigh (sp?) missile system)
Best Tank (post-1950) - Tie between T72 and M1A1.
Everyone simmer down a sec. The M1A1 for obvious reasons, but the T72? Sure, here's why: very inexpensive to manufacture, very well armored (for when it was fielded), excellent penetration, small, low profile, fast, dummy-proof, 125mm. The only drawback to the T72 is it's short engagement range and it's autoloader.